Unlike polymer electrolyte fuel cells, solid-oxide fuel cells (SOFCs) can use a wide variety hydrocarbon fuels (Solid State Ionics 135, 305 (2000), which is incorporated herein by reference). Because of their high operating temperatures (600 to 800° C.), metal catalysts added to the ceramic anodes can facilitate reforming reactions that generate H2 and CO from hydrocarbons. The conventional anode for an SOFC, a composite consisting of nickel and yttria-stabilized-zirconia (YSZ), has excellent catalytic activity for fuel oxidation, good conductivity for current collection, and unmatched compatibility with YSZ electrolyte for easy cell fabrication, but it is highly susceptible to carbon buildup (coking) and deactivation (poisoning) by contaminants commonly encountered in readily available fuels (Nature Mater. 3, 17 (2004), which is incorporated herein by reference). Some contaminants (e.g., sulfur impurities) can dramatically degrade its performance even at parts per million (ppm) levels (Science 312, 1508 (2006), which is incorporated herein by reference). Sulfur adsorbs strongly on Ni surface and thus blocks the active sites for electrochemical oxidation of fuel, resulting in considerably increased anodic polarization and energy loss.
To overcome these problems, significant efforts have been devoted to the development of new anode materials and novel electrode structures. For example, the use of a ceria-based anodes demonstrate the potential for direct utilization of methane in an SOFC (Nature 400, 649 (1999), which is incorporated herein by reference). Later, the use of a composite anode consisting of copper and ceria led to successful operation of an SOFC with higher hydrocarbons than methane, which are more prone to coking due to higher content of carbon (Nature 404, 265 (2000), which is incorporated herein by reference). However, some practical issues still remain: The low melting point of Cu makes it difficult to fabricate anode-supported cells using conventional co-firing ceramic methods and the poor catalytic activity of Cu for fuel oxidation limits cell power output. In another approach, a catalyst layer (e.g., Ru-ceria) was applied to a conventional Ni-YSZ anode to allow internal reforming of hydrocarbons. The effectiveness of this cell structure was confirmed for direct use of iso-octane without coking in an SOFC with power densities of 0.3 to 0.6 W cm−2 at 670 to 770° C. (Science 308, 844 (2005), which is incorporated herein by reference). Although this cell design has demonstrated the possibility of a simple low-cost SOFC system with common automotive fuels, the drawbacks include decreased power density, difficulty in current collection, and the high cost of Ru.
Nickel-free conducting metal oxides have also been developed as anode materials, including La0.75Sr0.25Cr0.5Mn0.5O3-δ, (with a Ce0.8Gd0.2O2-δ interlayer) (Nature Mater. 2, 320 (2003), which is incorporated herein by reference), Sr2Mg1-xMnxMoO6-δ (x=0-1) (Science 312, 254 2006, which is incorporated herein by reference), and doped (La,Sr)(Ti)O3 (Nature 439, 568 (2006) and Solid State Ionics 149, 21 (2002), which are incorporated herein by reference). These anode materials showed different degrees of improved tolerance to coking, re-oxidation, and/or sulfur poisoning under various SOFC operating conditions. In many cases, however, the power densities of the SOFCs using Ni-free oxide anodes are less than those demonstrated by conventional Ni-YSZ supported SOFCs with thin electrolytes (by more than 50%). This low efficiency arises from difficulties in fabricating thin-YSZ electrolyte on porous oxide anode supports that arise from delamination or formation of undesirable phases. In some cases, inadequate lateral conductivity (or substantial sheet resistance) of Ni-free oxide anodes also contributes to low power density, especially for SOFC designs with long current collection paths as in fuel cell stacks.
Thus, there is a need in the industry to develop new composition and structures.